1. Field of the Invention
This invention pertains generally to novel proton conducting membranes (PCMs). Novel PCMs are disclosed that comprise a host polymer, one or more oxide fillers, and a strong acid (pKa less than about 5). More particularly, the subject invention comprises PCMs containing Polymer-x-strong acid-y-filler oxide and like composites, wherein x is between about 1 and about 10 (with x as the molar ratio of acid anion to polymer repeat unit) and y≦about 50% (with y the weight percentage of filler oxide in the composite). More particularly, the host polymer is PVP or equivalent polymer, the strong acid is phosphoric acid or an equivalent acid with a pKa less than about 5, and the filler oxide comprises SiO2 or similar oxides.
2. Description of Related Art
Generally, the subject invention is utilized as a major component of a polymer electrolytic fuel cell (PEFC). PEFCs are generally comprised of three major components: the anode; the proton conducting membrane (PCM, the subject invention area); and the cathode. The PCM plays a critical role of transporting a proton from the anode to the cathode. It has to be highly proton conductive and also mechanically, thermally, and chemically stable. Water is produced at the interface between the cathode and the membrane. This water can be problematic, as discussed below, in operation of a PEFC. Lack of suitable membrane availability has been hindering the commercialization of PEFC. Water management is one of the most difficult issues in operating a PEFC. The water in the PEFC is produced as a product at the cathode side in PEFC. A breakdown in water balance between production and loss of water at the cathode side often results in water flood, while the anode interface with the membrane may suffer from water depletion due to water transportation toward the cathode side. Both the flood and the depletion may increase the cell over-potential which results in loss of power. Furthermore, the most commonly used PCMs are based on sulfonated perfluoropolymers that need to be fully humidified to be functional during the operation of the PEFC. Thus, these sulfonated perfluoropolymers not only require a humidifier, but also need an even distribution of water across the membrane which becomes an additional concern because of the membrane's high dependence on water.
Dry operation of PEFC may alleviate some of the water management problems. In fact, there is a strong demand in the auto industry as well as the distributed power generation industry for PEFC functional under low relative humidity (RH) (<50% RH)[Mathias, M.; Gasteiger, H.; Makharia, R.; Kocha, S.; Fuller, T.; Xie, T.; Pisco, J. Preprints of Symposia-American Chemical Society, Division of Fuel Chemistry 2004, 49(2), 471–474] Currently, no commercially available PCM meets this demand. NAFION, the industrial standard PCM by DuPont, is widely used in PEFC; yet it is sensitive to humidity, a very undesirable characteristic. Other existing proton conducting membranes, commercially available or under development, are as good or even better than NAFION under fully humidified condition, but very few outperform NAFION under low humidity conditions.
One existing PCM is disulfonated poly(arylene ether sulfone) copolymer (BPSH) developed by McGrath and coworkers[Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231] Though BPSH is thermally stable and mechanically durable, and widely used as one of the most advanced alternative PCM, its proton conductivity under low RH (<80%) is lower than that of NAFION. Lack of membranes capable of functioning under low RH, (i.e., maintaining high conductivity, ˜10−1 Scm−1) has been an obstacle to bringing PEFC to market. The challenge for the industry is how to improve the conductivity of PCMs, where water plays a vital role in proton transportation, under dry condition.
A typical approach previously attempted to improve the conductivity of PCMs under low RH has been to increase the degree of sulfonation in the PCM in an attempt to increase the overall conductivity. [Tchatchoua, C.; Harrison, W.; Einsla, B.; Sankir, M.; Kim, Y. S.; Pivovar, B.; McGrath, J. E., Preprints of Symposia-Am. Chem. Soc., Div. of Fuel Chem. 2004, 49(2), 601] The problem with such an approach is that the membrane tends to swell more with a higher degree of sulfonation, which is detrimental in operation of fuel cell since the dimensional stability of the PCM is a key to the operation. Also, there is synthetic difficulty associated with increasing degree of sulfonation. Furthermore, there is a theoretical limit to the conductivity due to the sulfonyl groups (—SO3H) in the membrane.
An existing alternative approach to improve proton conductivity is a fabrication of composite membranes based on the conventional water-based PEM and inorganic/organic additives including SiO2 and heteropolyacids (HPA). [Shao, Z-G.; Joghee, P.; Hsing, I-M. J. Membr. Sci. 2004, 229, 43] Especially, HPA has been widely used to improve the performance of proton conducting membranes[Herring, A. M.; Turner, J. A.; Dec, S. F.; Sweikart, M. A.; Malers, J. L.; Meng, F.; Pern, J.; Horan, J.; Vernon, D. Abst. 228th Am. Chem. Soc. National Meeting, Philadelphia, Pa., Aug. 22–26, 2004 FUEL-053] The problems with HPA, however, are that it is water-soluble, thus leaches out, and the proton conductivity is sensitive to humidity. [Katsoulis, D. E. Chem. Rev. 1998, 98, 359] Hence, immobilization of HPA in a membrane is a particularly important issue. [Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2003, 212, 263]
An existing and more radical approach to improve proton conductivity is to replace water altogether. PCM with low volatile solvents such as imidazole have been utilized to replace water. [Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, Maier, M. J. Electrochim. Acta 1998, 43, 1281] Though the proton conductivity of 10−2 Scm−1 has been achieved at high temperatures, imidazole is known to poison the Pt catalyst and also is subject to diffusing out of the membrane, which is currently fixed through chemical attachment to a host polymer. [Schuster, M. F. H.; Meyer, W. H.; Schuster, M.; Kreuer, K. D. Chem. Mater. 2004, 16, 329.] Also, work exists in which a polybenzimidazole membrane was doped by H3PO4 (PBI/H3PO4). [Fontanella, J. J.; Wintersgill, M. C.; Wainright, J. S.; Savinell, R. F.; Litt, M. Electrochimica Acta 1998, 43, 1289.] Yet, H3PO4 is known to be leached out by water on the cathode side. Improvement of the performance of a PBI/H3PO4 membrane has been achieved through the use of polyphosphoric acid, however, the poor performance at low temperature and leaching out of H3PO4 by water condensation remain unsolved. [Zhang, H.; Chen, R.; Ramanathan, L. S.; Scanlon, E.; Xiao, L.; Choe, E-W.; Benicewicz, B. C. Prep. Div. Fuel Cehm. Am. Chem. Soc., Philadelphia, Pa., Aug. 22–26, 2004, 49, 588.] In another approach to replace water, inorganic solid acids such as CsHSO4 have been used. [Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Nature (London, United Kingdom) 2001, 410, 910.] However, there are concerns regarding this solid acid: reduction of the sulfur in the CsHSO4 electrolyte may occur over time, the reaction with hydrogen forms hydrogen sulfide, and also a poisoning to the Pt catalyst may occur. Other solid acids may be less problematic, but the stability of the materials remain problematic since the operation temperatures for these solid acids are close to their thermal decomposition temperatures. Thus, anhydrous (non-water) membranes have not reached a practical stage for operation of PEFC.
Additionally, four existing membrane works should be mentioned. The first concerns poly(vinylidene fluoride) (PVDF)-H3PO4-silica hybrid membranes. [Carriere, D.; Barboux, P.; Chaput, F.; Spalla, O.; Boilot, J. P. Solid State Ionics 2001, 145, 141] The hybrid membrane was prepared in a manner vaguely related to the subject invention (i.e., a polymer/H3PO4 blend is integrated in a silica network). The proton conductivity reported was 5×10−3 Scm−1 under 20% RH at 30° C. when 0.2 mols of H3PO4 was mixed for every repeating unit of PVDF. The conductivity found in that membrane is lower than the best performance of the subject invention (see Example 2 below), but the content of H3PO4 is lower than with the subject invention.
The second membrane work is by Honma et al., who have presented reports on hybrid membranes based on a silica network. [Honma, I.; Nomura, S.; Nakajima, H. J. Membr. Sc. 2001, 185, 83 and Honma, I.; Nakajima, H.; Nishikawa, O.; Sugimoto, T.; Nomura, S. J. Electrochem. Soc. 2003, 150, A616] Their membranes were comprised of a polymer such as poly(ethylene oxide), poly(propylene oxide), and poly(tetramethylene oxide), 12-phosphotungstic acid, and SiO2. It is critical that all of their membranes needed to be fully humidified to give high conductivity, ˜10−2 Scm−1, and lost conductivity under low humidity conditions.
The third previously reported membrane work is by Chen et al. who reported a blend membrane of PVP and PVDF which achieved the conductivity of 1 0–2 5 cm−1 at room temperature, yet the membrane was fully humidified. [Chen, N.; Hong, L. Solid State Ionics, 2002, 146, 377]
Lastly, PVP has also been used as a PCM in combination with polyphosphoric acid. [Bozkurt, A.; Meyer, W. H. J. Polym. Sci. 2001, 39, 1987] However, the conductivities at room temperature and 120° C., 10−3 and 5×10−3 Scm−1, respectively, were both lower than the subject invention produced at room temperature.
It is noted and stressed that the subject approach is not to shun a way from water, the best medium for proton conduction, nor increasing the degree of sulfonation or using HPA. One of the host polymers, poly(vinyl pyrrolidone) (PVP), is very unique in that it has an amphipathic character (i.e., both hydrophilic and hydrophobic) due to the highly polar amide group and the carbonyl group for the hydrophilic portion and the non-polar hydrocarbon groups for the hydrophobic portion. It is known to form solid-state complexes with a wide variety of substances and the complexes are molecularly dispersed, suggesting this polymer is an ideal host polymer. [Carriere, D.; Barboux, P.; Chaput, F.; Spalla, O.; Boilot, J. P. Solid State Ionics 2001, 145, 141 and Honma, I.; Nomura, S.; Nakajima, H. J. Membr. Sc. 2001, 185, 83] Its high affinity toward acids, can accommodate a large amount of acid such as H3PO4 in the system and yet the acid is very tightly held in the interpenetrating network formed by the polymer, and associate oxide, such as SiO2. This leads to a high proton conductivity of the membrane and also to very little loss of the acid out of the membrane. The subject system (with one preferred example having PVP/H3PO4/SiO2 components, may vaguely resemble the PBI/H3PO4 system above. Yet, the subject system membrane is fundamentally and dramatically different in that it uses water, and also the acid, such as, but not limited to, H3PO4, for proton conduction, and thus maintains a high conductivity even at relatively low temperatures, <20° C., at which PBI/H3PO4 shows a poor performance. Furthermore, a fuel cell with the PBI/H3PO4 membrane needs to be maintained at high temperatures, >160° C., at which new concerns arise such as corrosion of the electrode. In other words, the subject invention allows for fuel cell operation at moderately low and high temperatures without external humidification and without presenting the problems associated with the currently available alternative PCMs. Further, it is stressed that the subject invention utilizes the filler oxide (e.g. SiO2) to form an interpenetrating network which is one of the fundamental differences from the PBI/H3PO4 system, in addition to using H2O. In the PBI/H3PO4 system, H3PO4 is also trapped in the network. Furthermore, the subject invention PVP-H3PO4—SiO2 even functions at subzero temperatures, which is another advantage over the PBI/H3PO4 system.
It is recorded that SiO2 has been used previously by mixing its particles in a PCM. [Shao, Z-G.; Joghee, P.; Hsing, I-M. J. Membr. Sci. 2004, 229, 43] However, the subject invention uses oxides, such as SiO2, to form a interpenetrating network in the membrane. This network not only enhances the mechanical property of the membrane, but also traps the selected acid, such as H3PO4, to avoid leaching out of the acid.
The subject hybrid membrane has higher conductivity than the prior arts. This is because the subject membrane has a different host polymer, PVP, which is a more desirable host polymer for H3PO4 due to PVP's extreme high affinity to the acid, thus having a higher uptake of H3PO4, leading to a higher conductivity even under low humidity. Also, it forms a very transparent, well dispersed membrane with very few pores.
In relation to Chen et al. [Chen, N.; Hong, L. Solid State Ionics, 2002, 146, 377], the subject invention uses H3PO4 as an extra proton carrier to facilitate the proton conduction and H3PO4 is imbedded in the silica network.
Hence, in the subject invention the host polymer, such as PVP, has a strong interaction with the selected acid, such as H3PO4, thus having a large intake of the acid. The acid is immobilized in the interpenetrating network formed by condensation of silica in the membrane to avoid leaching out. The membrane is flexible, transparent, and ductile. Because of a large amount of acid, such as H3PO4, already imbedded in the membrane, a small amount of water can significantly facilitates the proton conduction, thus leading to a high conductivity under a low humidity. The conduction mechanism is a proton being transferred between acid molecules or between an acid molecule and a water molecule or between water molecules in a combination of a Grotthus-type mechanism (i.e., the protons are transferred within adjacent hydrogen bonds) and a vehicular mechanism.
An approach to produce an effective PCM is based on organic-inorganic materials generated by sol-gel and has recently gained increasing interest in view of obtaining new composite compounds with unique properties that result from intimate mixing of various components on a molecular level. In the sol-gel process organic groups are introduced by co-condensation of alkoxides (of silicon, aluminum, titanium, zirconium, tin, etc.) with reactive organic functionalities followed by organic cross-linking reactions, thus providing the advantages of the polymers, while the inorganic network gives an amorphous structure and good thermal, mechanical and chemical stability. Some of the membranes investigated so far are: 1) NAFION/alkoxide composite membranes, such as, NAFION/SiO2—P2O5—ZrO2 [M. Aparicio, F. Damay, L. C. Klein, “Characterization of SiO2-P2O5-ZrO2 sol-gel/NAFION composite membranes”, Journal of Sol-Gel Science and Technology, 26(1/2/3) (2003), p. 1055–1059 and Aparicio, M.; Klein, L. C.; Adjemian, K. T.; Bocarsly, A. B., “SiO2-P2O5-ZrO2 sol-gel/NAFION composite membranes for PEMFC”, Ceramic Transactions, 127 (2002), p. 167–176], NAFION/silicon oxide [Jung, D. H.; Cho, S. Y.; Peck, D. H.; Shin, D. R.; Kim, J. S., “Performance evaluation of a Nafion/silicon oxide hybrid membrane for direct methanol fuel cell”, Journal of Power Sources, 106 (2002), p. 173–177 and Kim, Haekyoung; Lim, C.; Chang, H., “Fabrications and direct methanol fuel cell applications of Nafion based organic-inorganic hybrid membrane”, Proceedings-Electrochemical Society (2001), 2001–4(Direct Methanol Fuel Cells), p14–28], NAFION/glass [Nogami, M.; Usui, Y.; Kasuga, T., “Proton conducting organic-glass composites”, Fuel Cells, 1 (2001), p. 181–185], NAFION/silica [Miyake, N.; Wainright, J. S.; Savinell, R. F., “Evaluation of a sol-gel derived Nafion/silica hybrid membrane for polymer electrolyte membrane fuel cell applications 11. Methanol uptake and methanol permeability”, Journal of the Electrochemical Society, 148 (2001), A905–A909 and Miyake, N.; Wainright, J. S.; Savinell, R. F., “Evaluation of a sol-gel derived Nafion/silica hybrid membrane for proton electrolyte membrane fuel cell applications 1. Proton conductivity and water content”, Journal of the Electrochemical Society, 148 (2001), A898–A904], NAFION/TiO2 [Uchida, Hiroyuki; Ueno, Yoshihiko; Hagihara, Hiroki; Watanabe, Masahiro, “Self-humidifying electrolyte membranes for fuel cells”, Journal of the Electrochemical Society, 150 (2003), A57–A62], NAFION/Cs+ ions [V. Tricoli, “Proton and methanol transport in poly(perfluorosulfonate) membranes containing Cs+ and H+ cations”, J. Electrochem. Soc. 145 (1998), p. 3798–3801], and NAFION/alkoxides of Ti, Al, Zr and organoalkoxysilanes [V. Tricoli, “Proton and methanol transport in poly(perfluorosulfonate) membranes containing Cs+ and H+ cations”, J. Electrochem. Soc. 145 (1998), p. 3798–3801]. It was found that the incorporation of inorganic phase in NAFION led to improvements in its thermal stability, reduction of methanol crossover, and higher performance at elevated temperature. However, the proton conductivity in the hybrid membrane was lower than, or equal to, that in unmodified NAFION membranes if the inorganic phase was not P2O5 [M. Aparicio, F. Damay, L. C. Klein, “Characterization of SiO2-P2O5-ZrO2 sol-gel/NAFION composite membranes”, Journal of Sol-Gel Science and Technology, 26(1/2/3) (2003), p. 1055–1059, Nogami, M.; Usui, Y.; Kasuga, T., “Proton conducting organic-glass composites”, Fuel Cells, 1 (2001), p. 181–185, and Miyake, N.; Wainright, J. S.; Savinell, R. F., “Evaluation of a sol-gel derived Nafion/silica hybrid membrane for proton electrolyte membrane fuel cell applications 1. Proton conductivity and water content”, Journal of the Electrochemical Society, 148 (2001), A898–A904], which favored proton migration based on the Grotthus-type mechanism [T. Yajima and H. Iwahara H. Uchida, “Protonic and oxide ionic conduction in BaCeO3-based ceramics—effect of partial substitution for Ba in BaCe0.9O3— with Ca”, Solid State Ionics, 47 (1991), p. 117–124, N. Bonanos, B. Ellis, K. S. Knight and M. N. Mahmood, “Ionic conductivity of gadolinium-doped barium cerate perovskites”, Solid State Ionics, 35 (1989), p. 179–188, and H. Iwahara, H. Uchida, K. Morimoto, “High temperature solid electrolyte fuel cells using perovskite-type oxide based on barium cerium oxide (BaCeO3)”, J. Electrochem. Soc., 137 (1990), p. 462–465]. The studies suggested that inorganic network led to increased close “channel” in film formation, thus decreased the ionic conductivity. 2) Polymer/heteropolyacid composite membranes [Chu, Young-Hwan; Lim, Jung-Eun; Kim, Hyun-Jong; Lee, Chang-Ha; Han, Hak-Soo; Shul, Yong-Gun, “Proton conducting silica mesoporous/heteropolyacid-PVA/SSA nano-composite membrane for polymer electrolyte membrane fuel cell”, Studies in Surface Science and Catalysis, 146 (2003), (Nanotechnology in Mesostructured Materials), p. 787–790, Honma, I.; Nakajima, H.; Nishikawa, O.; Sugimoto, T.; Nomura, S., “Family of High-Temperature Polymer-Electrolyte Membranes Synthesized from Amphiphilic Nanostructured Macromolecules”, Journal of the Electrochemical Society, 150 (2003), A616–A619, Honma, Itaru; Nakajima, Hitoshi; Nishikawa, Osamu; Sugimoto, Toshiya; Nomura, Shigeki, “Proton conducting electrolyte membranes synthesized through amphiphilic organic/inorganic hybrid macromolecules”, Electrochemistry (Tokyo, Japan), 70 (2002), p. 920–923, Honma, Itaru; Nakajima, Hitoshi; Nomura, Shigeki, “High temperature proton conducting hybrid polymer electrolyte membranes”, Solid State Ionics, 154–155 (2002), p. 707–712, Honma, I.; Nakajima, H.; Nishikawa, O.; Sugimoto, T.; Nomura, S., “Amphiphilic organic/inorganic nanohybrid macromolecules for intermediate-temperature proton conducting electrolyte membranes”, Journal of the Electrochemical Society, 149 (2002), A1389–A1392, Honma, I.; Nishikawa, O.; Sugimoto, T.; Nomura, S.; Nakajima, H., “A sol-gel derived organic/inorganic hybrid membrane for intermediate temperature PEFC”, Fuel Cells, 2(1) (2002), p. 52–58, Nakajima, Hitoshi; Honma, Itaru, “Proton-conducting hybrid solid electrolytes for intermediate temperature fuel cells”, Solid State Ionics, 148(3,4) (2002), p. 607–610, Lavrencic Stangar, U.; Groselj, N.; Orel, B.; Schmitz, A.; Colomban, Ph., “Proton-conducting sol-gel hybrids containing heteropoly acids”, Solid State Ionics, 145(1–4) (2001), p. 109–118, and Honma, I.; Takeda, Y.; Bae, J. M., “Protonic conducting properties of sol-gel derived organic/inorganic nanocomposite membranes doped with acidic functional molecules”, Solid State Ionics, 120(1–4) (1999), p. 255–264]. By doping heteropolyacid like 12-phosphotungstic acid (PWA) into the network and through sol-gel processing of bridged polysilsesquioxanes, amphiphilic organic/inorganic nano-hybrid membranes were synthesized. The membranes show large proton conductivity at intermediate temperature up to 140° C. and flexible as well as thermally stable due to the temperature tolerant inorganic frameworks. 3) Other organic/inorganic hybrids, such as, PVA/silica [Kim, Dae Sik; Shin, Kwang Ho; Park, Ho Bum; Rhim, Ji Won; Lee, Young Moo., “PVA/silica hybrid membrane containing sulfonic acid group for direct methanol fuel cells application”, Membrane, 13(2) (2003), p. 101–109], polyethylene glycol/SiO2 [Chang, H. Y.; Lin, C. W., “Proton conducting membranes based on PEG/SiO2 nanocomposites for direct methanol fuel cells”, Journal of Membrane Science, 218(1–2) (2003), p295–306 and Honma, I.; Hirakawa, S.; Yamada, K.; Bae, J. M., “Synthesis of organic/inorganic nanocomposites protonic conducting membrane through sol-gel processes”, Solid State Ionics, 118(1,2) (1999), p. 29–36], 3-glycidoxypropyltrimethoxy-silane/P2O5 [Tadanaga, Kiyoharu; Yoshida, Hiroshi; Matsuda, Atsunori; Minami, Tsutomu; Tatsumisago, Masahiro, “Proton conductive inorganic-organic hybrid membranes as an electrolyte for fuel cells prepared from 3-glycidoxypropyltrimethoxysilane and orthophosphoric acid”, Electrochemistry (Tokyo, Japan), 70(12) (2002), p. 998–1000], perfluorosulfonic acid/SiO2 [Adjemian, K. T.; Srinivasan, S.; Benziger, J.; Bocarsly, A. B., “Investigation of PEMFC operation above 100° C. employing perfluorosulfonic acid-silicon oxide composite membranes”, Journal of Power Sources, 109(2) (2002), p. 356–364], and polypropylene glycol/SiO2 [Honma, I.; Nomura, S.; Nakajima, H., “Protonic conducting organic/inorganic nanocomposites for polymer electrolyte membrane”, Journal of Membrane Science, 185(1) (2001), p. 83–94]. In general the composite membranes have higher mechanical and thermal stabilities and are more robust than the control membranes (unmodified membranes), which degrade after high operation temperature and thermal cycling. However, the ionic conductivity of the membranes is very dependent on the content of sulfonic acid group that works as a donor of hydrophilic SO3H group.